1. Field of the Invention
This invention relates to the field of calibration reticles for measuring microscopes.
2. Prior Art
Accurate, and even more important consistent, calibration of geographically separated measuring microscopes has become of great importance as a result of the wide diversification of the semiconductor industry. There are many mask production facilities scattered throughout the world which are in the business of fabricating masks to the order of customers. In some instances, linewidths in the finished masks are narrow enough and specified to a sufficient degree of accuracy that the tolerance allowed in the mask linewidth is less than the tolerance in the calibration of a measuring microscope whose calibration has been established using a prior art calibration reticle.
Prior art calibration reticles have utilized narrow scribed lines on the calibration reticle to define calibration distances. These calibration reticles induce operator error during calibration because the scribe lines, at calibration magnifications, are observed to be of varying width and ragged and because calibration using these reticles requires operator judgment as to where the edge of the line is. When calibrations are performed utilizing these prior art calibration reticles the calibration error can exceed the tolerance required of mask dimensions in fabricating state of the art semiconductor devices.
When a customer requiring a mask specifies a dimension and tolerance, the mask making facility may actually generate a mask having exactly the specified dimension as measured by the measuring microscopes in its quality control facility. However, because of differeing microscope calibration, the incoming inspection department of the customer may measure the masks as being out of tolerance.
A scribed line or other calibration reticle which utilizes the edge of a mark on the reticle as a calibration reference requires that the microscope be meticulously focused in order for accurate calibration to be achieved because even slight defocusing changes the apparent position of the edge of the mark.
A further problem with scribed line calibration reticles is the cost associated with need to individually calibrate the line spacing in order that the reticle will be an accurate calibration reference.
A calibration reticle is needed which minimizes the quantity of error introduced into a calibration via operator judgments and operator errors and consistenly yields calibrations having a tolerance which is less than the tolerance allowed in mask linewidths.